Newsgroups: comp.risks
X-issue: 1.37
Date: Thu, 9 Jan 86 12:14:34 PST
From: ihnp4!utzoo!henry@ucbvax.berkeley.edu
To: risks@sri-csl.arpa
Subject: Multiple redundancy
Advocates of multiple redundancy through independently-written software
doing the same job might be interested in an incident involving complete
failure of such a scheme.
During the development of the De Havilland Victor jet bomber, roughly a
contemporary of the B-52, the designers were concerned about possible
problems with the unusual tailplane design. They were particularly
worried about "flutter" -- a positive feedback loop between slightly-flexible
structures and the airflow around them, dangerous when the frequency of the
resulting oscillation matches a resonant frequency of the structure. So
they tested for tailplane flutter very carefully:
1. A specially-built wind-tunnel model was used to investigate the
flutter behavior. (Because one cannot scale down the fluid properties
of the atmosphere, a simple scale model of the aircraft isn't good
enough to check on subtle problems -- the model must be carefully
built to answer a specific question.)
2. Resonance tests were run on the first prototype before it flew,
with the results cranked into aerodynamic equations.
3. Early flight tests included some tests whose results could be
extrapolated to reveal flutter behavior. (Flutter is sensitive to
speed, so low-speed tests could be run safely.)
All three methods produced similar answers, agreeing that there was no
flutter problem in the tailplane at any speed the aircraft could reach.
Somewhat later, when the first prototype was doing high-speed low-altitude
passes over an airbase for instrument calibration, the tailplane broke off.
The aircraft crashed instantly, killing the entire crew. A long investigation
finally discovered what happened:
1. The stiffness of a crucial part in the wind-tunnel flutter model
was wrong.
2. One term in the aerodynamic equations had been put in wrongly.
3. The flight-test results involved some tricky problems of data
interpretation, and the engineers had been misled.
And by sheer bad luck, all three wrong answers were roughly the same number.
Reference: Bill Gunston, "Bombers of the West", Ian Allen 1977(?).
Henry Spencer @ U of Toronto Zoology
{allegra,ihnp4,linus,decvax}!utzoo!henry